This invention generally relates to an apparatus for processing an image signal. This invention specifically relates to, for example, an apparatus for processing a true halftone (gray-level) image signal into a pseudo halftone image signal having a sequence of bi-level (two-level) pixel components.
Some of image signal processing apparatus use an error diffusion method in converting a true halftone (gray-level) image signal into a pseudo halftone image signal having a sequence of bi-level pixel components.
Japanese published unexamined patent applications 62-139472, 1-238373, and 1-302960 disclose such image signal processing apparatus.
Japanese published unexamined patent application 1-303869 discloses one type of such image signal processing apparatus. The apparatus of Japanese application 1-303869 has a smoothing matrix by which an input image signal is smoothed. By comparing the resultant (smoothed) image signal with a given value, a decision is made as to whether or not a pixel of interest exists in a white area. Bit correction of the result of an error diffusion process is performed in response to the result of the decision. Thereby, it is possible to remove 1-dot (black dot) noise which might occur in the white area due to the accumulation of errors during the execution of the error diffusion process.
As will be explained later, prior-art image signal processing apparatus have some problems.